In the digital transmission of binary information, the signals are generally converted to multilevel signals prior to transmission. The particular coding of the multilevel signal has a direct bearing on the bandwidth compression, the transmission efficiency, the cost and complexity of the equipment, the error performance and the difficulty of extracting clock or timing information.
For maximum efficiency, the multilevel symbol rate should be inversely proportional to the number of levels of the two signals. Thus, a 100% efficient quaternary code has a symbol rate equal to one-half the binary bit rate. If block mapping codes are used small coding blocks must be used during code conversion to reduce the complexity. In addition, the running digital sum of the transmitted signal should be constrained so that there is no d-c component, otherwise d-c restoration techniques will be required. The low frequency power of the transmitted signal should also be small in order that small components (particularly small coupling transformers) may be used throughout the system and to minimize the effects of impulse noise. To permit simple clock extraction from the received signal, it is desirable that the spectral energy of the transmitted code be non-zero at the Nyquist rate and zero at twice the Nyquist rate. In addition, the transmitted code must contain sufficient framing and error checking information to function correctly in the transmission system irrespective of the input bit sequence.
U.S. Pat. No. 3,753,113 entitled "Multilevel Code Signal Transmission System" issued Aug. 14, 1973 to Rikio Maruta et al, describes a system for transmitting multidigit words that include one digit (or symbol) which represents m-1 binary bits, as well as polarity inversion and synchronization information. The latter can be detected because one level (in this case the zero level) is a forbidden level and does not occur in this digit.
An entirely different approach is described in U.S. Pat. No. 3,754,237 entitled "Communication System Using Binary To Multilevel And Multilevel To Binary Coded Pulse Conversion" issued Aug. 21, 1973 to Patrick de Laage de Meux. In this system the binary signal is divided into words of n bits to which an (n+1)th bit of constant value is added before coding to a multilevel signal. The (n+1) bit words are then subdivided into partial words, each of which is translated into a multilevel pulse of one or the other polarity in order to constrain the running digital sum of the multilevel signal and hence eliminate the d-c component. Since the (n+1)th bit of each partial word is also inverted, this information can be utilized to correctly reconstruct the original word in the receiver. Also with this scheme, there is spectral energy at the Nyquist rate and none at twice the Nyquist rate thereby facilitating clock recovery. However, to obtain synchronization, an additional synchronization word is transmitted at periodic intervals. This synchronization word reduces the coding efficiency of this coding scheme over that which is obtained by adding only the (n+1)th bit of constant value to each word.
The related patent by Betts et al identified above described a split-block encoder in which an additional separate multilevel symbol is added to each group of words (which in itself does not include any binary signal information). This additional symbol is used to reconstruct the correct polarity of the original words in each frame, and in conjunction with the words to directly derive the block synchronization and framing information without the inclusion of a separate synchronization or framing word, thereby increasing the overall efficiency of the digital transmission system. Although this system exhibits a d-c null, there is still substantial energies at frequencies close to d-c. Because of the variable cable characteristics at low frequencies, it is desirable to utilize high-pass filtering to eliminate the signal components in this portion of the band. This filtering exacerbates the performance due to the rejected signal energy and hence there is a requirement for low spectral energy at low frequencies.